1. Field of the Invention
The present invention relates to a diaphragm device of a television camera lens for a CCTV (closed-circuit television) camera, used as a surveillance camera, which incorporates an optical filter such as an infrared absorbing filter. The present invention also relates to a diaphragm device of a television camera lens for a CCTV camera, used as a surveillance camera, which is available in the range of a visible light region (approximately from 400 to 700 nanometers) to a near-infrared region (approximately 700 to 1000 nanometers).
2. Description of the Related Art
A CCTV surveillance camera in which an image of light in the visible light region is formed on a color CCD in a camera body to be indicated as a color image on a monitoring screen in the daytime, and in which an image of light in the near-infrared region excluding visible light is formed on the same color CCD to be indicated as a monochrome image on the monitoring screen at nighttime, is known in the art. In such a CCTV surveillance camera, a near-infrared absorbing filter is positioned in front of the CCD in the camera body or the lens barrel in an imaging operation during the daytime so that a color-image taking operation is performed for light only in the visible light region, and the near-infrared absorbing filter is retracted from the front of the CCD during an imaging operation at nighttime so that a monochromatic image-forming operation is performed for light in both the visible light region and the near-infrared region.
FIGS. 9A and 9B show fundamental elements of a conventional CCTV surveillance camera which incorporates an infrared absorbing filter. This CCTV surveillance camera is provided with a camera body 10′ and a lens barrel 18′ fixed to the front of the camera body 10′. The camera body 10′ is provided therein with an infrared absorbing filter 22′. FIG. 9A shows a state where the infrared absorbing filter 22′ is positioned in front of a CCD 11 on an optical axis L of a television camera lens, while FIG. 9B shows a state where the infrared absorbing filter 22′ is retracted from the front of the CCD 11 on the optical axis L. The incident light on the lens barrel 18′ enters the camera body 10′ to be incident on the CCD 11 via the infrared absorbing filter 22′ to be indicated as an image on a monitoring screen (not shown) in the state shown in FIG. 9A, and the incident light on the lens barrel 18′ enters the camera body 10′ to be incident on the CCD 11 without passing through the infrared absorbing filter 22′ to be indicated as an image on the monitoring screen in the state shown in FIG. 9B. An actuator such as a galvanometer type actuator 24 drives a diaphragm device 21′ provided in the lens barrel 18′ to adjust the size of the aperture of the diaphragm device 21′. On the other hand, the infrared absorbing filter 22′ is driven by another actuator 25′ such as a motor provided in the camera body 10′ to be inserted into and retracted from an optical path in front of the CCD 11 on the optical axis L as shown in FIGS. 9A and 9B, respectively.
However, since the camera body of the CCTV surveillance camera, whose camera body and lens barrel are formed integral with each other, is generally small, it is difficult to incorporate a drive unit for the near-infrared absorbing filter into the small camera body.
A conventional diaphragm device in which an ND filter is adhered to a diaphragm blade to be positioned on the aperture formed by the diaphragm for the purpose of extending the range of control of the light quantity during an imaging operation at nighttime, i.e., during the time when the infrared absorbing filter 22′ is retracted from the front of the CCD, is known in the art. However, the transmittance of a conventional ND filter for infrared light is generally high. FIG. 10 shows the spectral characteristics of a conventional ND filter by way of example. During monochromatic image-forming, this type of ND filter cannot adequately control all the quantity of the incident light including visible light and infrared light, and the transmittance for infrared light becomes greater than that for visible light. As a result, a portion of an object which has a high reflectivity for infrared light appears so bright that an image of the portion is difficult to be seen on a monitoring screen.